This invention relates to a system for measuring the velocity of a spacecraft or of a satellite.
In a known method, the velocity of a spacecraft is measured by computing Doppler shift of a radio wave which is transmitted from the spacecraft to a station on the earth. In another known method the positions of a spacecraft relative to the earth are measured at different points of time on the basis of detected radio wave propagation time between the earth and the spacecraft, and then the velocity of the spacecraft is calculated from the obtained position data and the time intervals of the measurements. By these conventional methods, however, it is difficult to accurately measure the velocity of spacecraft in that these methods provide only a velocity vector in the line-of-sight direction. Besides, communication between the spacecraft and the earth is indispensable to the conventional methods, and therefore there is the need of providing and maintaining sophisticated ground receiver apparatus which in some cases have to include a transmitter and in every case require an antenna system of large scale.